(1) Field of the Invention
The present invention relates to (i) a switching power supply apparatus which: generates a regulated output direct current voltage by causing a switching element to switch an input direct current voltage on and off, and (ii) a semiconductor device used for switching power supply control.
(2) Description of the Related Art
Switching power supply apparatuses equipped with semiconductor devices have been widely used for household equipment, such as electric appliances, as its power supply apparatuses in order to reduce its electric consumption and improve its power efficiency. The semiconductor devices utilize a switching operation performed by a switching element, such as a transistor, to control an output voltage.
Recently, in particular, global warming prevention measures have seen a significant attention to the reduction of the stand-by electricity that the apparatuses, such as the household appliances, require. This generates a great demand for a switching power supply apparatus consuming less stand-by electricity.
In most cases, a typical energy loss of a switching power supply under the light load, such as a stand-by load, is due to a switching loss caused by the switching operation. One of the techniques to improve power efficiency under the light load is to employ intermittent oscillation control under the light load to operate the switching power supply apparatus.
FIG. 1 is a block diagram exemplifying a structure of a switching power supply apparatus 800 including a semiconductor device having a conventional intermittent oscillation control circuit.
FIG. 2 is a timing diagram exemplifying an operation of the switching power supply apparatus 800.
Described briefly below with reference to FIGS. 1 and 2 is an operation on the switching power supply apparatus 800 during an intermittent oscillation period. The switching power supply apparatus 800 is assumed to execute pulse width modulation (PWM) control of a current mode at, for example, a switching frequency of 100 kHz during a normal operation.
In the switching power supply apparatus 800 shown in FIG. 1, as the load current Iout, having been outputted since the load rated state shown in FIG. 2, decreases under the load varying state, the output voltage Vout increase. A feedback signal (a current IFB which increases as the output voltage Vout increases) is inputted from an output voltage detecting circuit 5 to an FB terminal. Based on a degree of the current IFB, a feedback control circuit 11 outputs a control signal VEAO indicating a smaller limiting value with respect to a current ID flowing into a switching element 2 as the output voltage Vout is greater. A further decreasing load current Iout activates an intermittent oscillation control circuit 16 to cause the intermittent oscillation control circuit 16 to input an Enable signal into a turn-on control circuit 18, the Enable signal which alternatively indicates suspension and execution of the switching operation. This switches the switching power supply apparatus 800 into the intermittent oscillation control, as shown in a first stand-by state in FIG. 2, which involves the suspension and execution of the switching operation on the switching element 2. Moreover, an output load current smaller than that observed in the first stand-by state initiates a second stand-by state having a longer suspension period than the first stand-by period has. In other words, the blocking cycle, including an execution period and a suspension period of the switching operation on the switching element 2, is controlled to be longer as the load is lighter. Hence, the switching power supply apparatus 800 improves in power supply efficiency under the light load, using the intermittent oscillation control performed under the light load.
During the normal operation, the above switching operation is executed at 100 kHz. During the intermittent oscillation control state, however, the frequency in the oscillation control, which involves switching between the execution period and the suspension period of the switching on the switching element 2, is for example 20 kHz or below representing an audio-frequency range. As a result, noise may be generated from a transformer or a condenser typically used for the switching power supply. In addition, a lower frequency for the blocking control causes greater output ripple and the resulting deterioration in stability of the output voltage, which may results in failure to meet power supply specifications.
In other words, performing the intermittent oscillation control under the light load is effective in improving power supply efficiency under the light load; however, this lowers the frequency of the breaking control to the audio-frequency range, which causes drawbacks such as the noise from the transformer and the greater output ripple.
One of known counter measures is to lower a peak current value of the switching element during the intermittent oscillation period. Excessively lowering the peak current value in the intermittent oscillation increases the number of switching times during the intermittent oscillation, which makes improvement in the power supply efficiency rather ineffective. In other words, determined by the peak current value of the switching element during the intermittent oscillation is trade-off relationship between noises caused by a transformer and output ripple.
The optimum peak current value of the switching element during the intermittent oscillation period varies in different switching power supply apparatuses since (i) a certain degree of peak current value can be tolerated in the case where the noise can be reduced by using a bonded or an impregnated transformer, and (ii) a peak current value needed to avoid excessive increase in the number of switching times in the intermittent oscillation varies depending on an assumed load. Thus, the peak current value of the switching element in the intermittent oscillation can be desirably set easily at the choice of a designer of the switching power supply apparatus.
Non-Patent Reference 1 (Fuji Electric Co., Ltd. Catalogue “Fuji Switching Power Supply Control IC FA5540/5541/5542 Application Note” (July 2007)) discloses an effective switching power supply apparatus which meets the above demands.
FIG. 3 is a block diagram showing a structural example of a switching power supply apparatus 900 built with an application of the technique disclosed in Non-Patent Reference 1 to the conventional switching power supply apparatus 800.
The switching power supply apparatus 900 is the switching power supply apparatus 800 additionally including a structure disclosed in Non-Patent Reference 1. In the structure, a constant current source 444 is incorporated in a switching element current detecting terminal IS, and a constant current flows from the switching element current detecting terminal IS.
An offset adjusting resistor 45 is inserted between (i) a switching element current detecting circuit (current detecting resistor 46) detecting a current flowing through the switching element 2 and (ii) the switching element current detecting terminal IS. Here, an offset effect is observed due to a voltage generated by the product of a value of the offset adjusting resistor 45 and a current value of the constant current source 444. Because of the offset effect, a greater resistor value of the offset adjusting resistor 45 nominally provides a greater switching element current detecting value.
This allows a power supply apparatus designer to easily adjust the peak current value of the switching element 2 during the intermittent oscillation period with a use of the resistor value of the offset adjusting resistor 45, which makes possible realizing a switching power supply apparatus executing the most suitable intermittent oscillation control under the light load.